Test pieces preparation

An acceptable middle way is to cut test pieces from the product. This has the advantage that the properties measured relate to the material as processed in the factory rather than to test pieces prepared under laboratory conditions. The only disadvantage is the limitation in obtaining suitable test pieces from many products. 

Whatever the other considerations may indicate, it is a simple fact of life that in virtually all trials the selection of properties will be affected by the cost and convenience of the experimental requirements. There are enormous differences between different properties in the cost of test piece preparation, testing time, number and size of test pieces, and apparatus requirements. In accelerated exposures the availability of exposure space is very frequently the limiting factor. 

ISO 20182 2005 Refractory test-piece preparation - gunning refractory panels by pneumatic-nozzle mixing-type guns... 

Figure 5.10 Tensile test piece preparation with fragile nanofiber mats. Redrawn from Huang et al. (2004),...

In cases where test pieces (or items) are prepared, the issue of obtaining a homogeneous batch of items is even more complex. Here the preparation procedure sets limits, in combination with the properties to be certified. The uncertainty of the property values should appreciate this fact, as otherwise the uncertainty of the reference material is only valid for the batch, not for a single item from the batch. This... 

The test piece shall be large enough to permit preparing three specimens from the weld metal and three from the HAZ (if required) per para. GR-2.1.3. If this is not possible, preparation of additional test pieces is required. 

There are occasions when a product test will cost no more than the preparation and testing of pieces cut from it. This would be true, for example, for impact testing of a bucket because the cost of making test pieces would be little different from the value of the container itself and the testing costs would be equal. Impacting a bucket would actually be cheaper than cutting standard impact test pieces from it. 

The number and type of test pieces exposed for each measurement point will depend on the property being measured. If measurement is non-destructive, e.g., loss of mass or colour, then the same specimens can be used throughout the test, being replaced in the oven after each measurement. If the measurement is destructive then one set of specimens must be prepared for each combination of duration and temperature. It is recommended to expose at least two reserve sets in case the threshold level has not been reached by the end of the last planned duration. Usually the number specified in the relevant test method standard is chosen but, again, the more the better. An example and some of the problems are described in Section 12.2. 

The methods for the preparation of test pieces (or products) and their subsequent history will influence the material properties. Consequently, it is important for any testing that all the moulding, storage, preparation and conditioning procedures are defined and carefully controlled. It may be worthwhile conducting preliminary trials. 

ISO 4661-2 1987 Rubber, vulcanized - Preparation of samples and test pieces - Part 2 Chemical tests... 

The properties of a material and, hence, the test results obtained will depend on the processing used to produce the test material or product. Frequently, this is beyond the control of the tester and he or she is only required to characterise the samples received. Nevertheless, if any controlled comparison is to be made it is essential that the test material is produced in exactly the same way. Similarly, any preparation which is performed on the material to produce the test pieces is likely to influence the results. Consequently, it is highly desirable that preparation is standardised and comparisons only made between test pieces produced in the same way, including the direction within the sheet that test pieces were cut. Standard procedures are discussed in chapter 4. 

Except for work on complete products, a test piece must be formed before the test can be carried out. In many cases, the test piece can be directly moulded but, particularly when tests on finished products are concerned, the specimens need to be cut and/or buffed to some particular geometric shape (Figure 4.1). It is convenient to consider separately, first the mixing and moulding leading up to a vulcanised (or thermoplastic) test piece or test sheet, and secondly the preparation of test pieces from moulded sheets or products. The preparation of test pieces for tests on raw rubber and unvulcanised compounds will be considered integrally with those tests in Chapter 6. 

Processing variables can affect to a very great extent the results obtained on the rubber product or test piece and, in fact, a great number of physical tests are carried out in order to detect the result of these variables, for example state of cure and dispersion. In a great many cases, tests are made on the factory prepared mix or the final product as it is received but, where the experiment involves the laboratory preparation of compounds and their moulding, it is sensible to have standard procedures to help reduce as far a possible sources of variability. Such procedures are provided by ISO 2393 which covers both mills and internal mixers of the Banbury or Intermix type, and also procedures for compression moulding. 

Although it is debatable whether mixing and moulding are strictly part of testing, particularly as these processes are often not under the control of the tester, there is no doubt that the preparation of test pieces from moulded sheet or products is part of the testing process. The most common operation... 

To stamp, for example, a dumb-bell from sheet requires only a die and a press, although a hammer has been known to replace the latter. There has been a tendency to treat stamping as so simple an operation as to merit little attention, despite the fact that the accuracy of the final test result depends very considerably on the accuracy with which the test piece was prepared. The necessary dimensions of the die are given in the relevant test method standard, for example ISO 375 for tensile properties, but there is ISO 235292 which deals specifically with the preparation of test pieces for physical tests. ISO 23529 now incorporates what was ISO 4661 Part 1 but ISO 4661 Part 2 still exists and deals with the preparation of samples for chemical tests. The British standard is identical to the ISO standard7. 

When it is the product rather than the material that is being investigated, it is obviously desirable to make tests, wherever possible, on the actual finished product rather than on specially prepared test pieces, which may have been produced under rather different conditions. Apart from the difficulty of having sufficient bulk in the product to obtain standard test pieces, extra operations may be involved which are time consuming and are likely to lead to lower test results because of destruction of the moulded surface. However, these difficulties can often be overcome satisfactorily by the use of miniaturised test pieces and by careful use of cutting and buffing apparatus. 

ISO 23529, 2004. General procedures for preparing and conditioning test pieces for physical test methods. 

Mixing and moulding have been considered under the general heading of preparation of test pieces in Chapter 4. Where compounded but unvulcanised rubbers are to be tested, the same standard mixing procedures will be relevant together with further details relevant to particular polymers, as referred to in Section 1 above. 

More modem versions of the Defo test have vacuum preparation of the test piece and computerised control but although they measure both the viscous and elastic components, it is still a compression test at low shear rate. Isayev et al27 described an instrument and method to discriminate between materials by measuring the elastic recovery at very short times. 

A useful procedure for checking if test pieces lie within certain limits of density is to prepare two liquids of different but known densities to be within the known limits a test piece must sink in one liquid and float in the other. This can be employed, for example, to rapidly sort parts made in two materials which have been mixed up. A further variation6 is titration of a heavier liquid into a lighter liquid until the test piece just floats, as given in ISO 1183-16. 

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